The fundic mucosa which secretes both acid and pepsinogen into the gastric lumen plays an important role in the early digestion of food and in several pathophysiological states in humans. Much of our present knowledge of the control of gastric secretory processes by peptide hormones has been derived from in vivo and in situ models. While these studies have yielded a considerable amount of descriptive information, our understanding of the interactions between these hormones and specific receptor sites in the fundic gastric mucosa that initiate acid and pepsinogen secretion is quite limited. With the development of gentle and reliable methods to isolate viable preparations of gastric glands and enriched preparations of chief and parietal cells, it is now possible to define these initial interactions without the complicating influences found in vivo and in situ. It is the purpose of the present research proposal to characterize the interactions between 125I-labeled peptide derivatives of gastrin, CCK-8 and secretin, and their specific receptors in such cellular preparations. The binding of each radioligand to glands and cells will be assessed by well established criteria such as rapid rates of association, reversibility, saturability and specificity. Since the gastric gland contains several different cell types, the identification of binding sites for these peptides on the individual cells will be defined by light microscopic autoradiographic methods in tissue sections and dispersed glands, and by standard binding methods in separated enriched fractions of parietal and chief cells. The extent of internalization of these peptides after binding will be determined by reversibility studies, the fate of internalized peptides by gel-filtration, and the intracellular location of internalized peptides defined by electron microscopic autoradiography. The receptor subunit structures for gastrin, CCK and secretin will be characterized in isolated plasma membranes from gastric glands by affinity labeling methods which will require the preparation of a new class of 125I-labeled photoactivable derivatives of gastrin and CCK. By defining the early events in hormone control of the fundic gastric mucosa a much clearer understanding of the role of each peptide hormone in gastric secretory regulation will emerge.